1. Field of the Invention
This invention relates to the fabrication of solar cells, and in particular, to a method of removing short circuits in solar cell elements during manufacturing.
2. Description of the Related Art
As shown in FIG. 1, a thin film solar cell 10 comprises a plurality of solar cell elements 5a, 5b and 5c formed on an insulating substrate 1. Each solar cell element 5 comprises a first electrode 2 formed on one face of substrate 1 in a preset pattern; a semiconductor layer 3 for performing photoelectric conversion which is formed on the surface of the first electrode 2 and a second electrode 4 formed on the surface of the photoelectric-conversion semiconductor layer 3. The semiconductor layer 3 may consist of a non-crystalline semiconductor. The plurality of solar cell elements 5a, 5b and 5c are connected in series by connecting the first electrode 2a of solar cell element 5a to the second electrode 4b of neighboring solar cell element 5b, and the first electrode 2b of solar cell element 5b to the second electrode 4c of neighboring solar cell element 5c.
When a glass substrate, a transparent resin substrate or the like is used as the insulating substrate 1 of the solar cell, a transparent electrode material such as ITO (Indium Tin Oxide, indium oxide mixed with tin oxide) or the like is used as the first electrode 2, and a metal electrode material is used as the second electrode 4. When a nontransparent material is used as the insulating substrate 1, a metallic electrode material is used as the first electrode 2, and a transparent electrode material is used as the second electrode 4.
In the case where the semiconductor layer 3 is a non-crystalline silicon base semiconductor, non-crystalline silicon consisting of alloy of silicon and carbon or other metal such as germanium, tin, etc. may be used, as well as non-crystalline silicon, hydrogenated non-crystalline silicon, hydrogenated non-crystalline silicon carbide, or non-crystalline silicon nitride. Furthermore, these non-crystalline or polycrystalline semiconductor materials may be used in the form of pin-type, nip-type, ni-type, pn-type, MIS-type, heterojunction-type, homojunction-type, Schottky barrier-type, or a combination of the above. Also, the semiconductor layer may be formed by using not only a silicon base but also a Cd base, GaAs base, InP base, etc.
When, for example, a short-circuit occurs between the first electrode 2b and the second electrode 4b of solar cell element 5b by a pinhole formed in the photoelectric-conversion semiconductor layer 3b during manufacturing, it is well known to remove the short-circuited section or to insulate it by oxidation
When a short-circuited section to be removed is between the first electrode 2b on the substrate side of the solar cell element 5b and the second electrode 4b on the back side of the photoelectric-conversion semiconductor layer 3b, probe electrodes 6a and 6b contacting electrodes 4b and 4c, respectively, are used. A DC voltage or voltage of a square-wave pulse, not exceeding the reverse limit voltage (reverse breakdown voltage), is applied in the reverse direction (0 V side), as shown in FIG. 2, between the first electrode 2b and the second electrode 4b that sandwich the photoelectric-conversion semiconductor layer 3b. The electric current is concentrated at the short-circuited section which generates Joule heat. Oxidation of metal occurs due to the generated Joule heat. The oxidation of metal insulates the short-circuited section. Another method of removing the short circuit involves removing the short-circuited section by dissipation of the metal.
However, a solar cell is equivalent to a diode. Thus, when the voltage is applied in the reverse direction between the electrode 2 and the electrode 4, the solar cell element 5 consisting of the first electrode 2, the photoelectric-conversion semiconductor layer 3 and the second electrode 4 functions as a capacitor, and charges accumulate across the capacitor. As a result, when the DC voltage is applied between electrodes 2 and 4, charges remain between electrodes 2 and 4 even after the applied voltage has been removed abruptly. The voltage generated by these charges may cause electrical breakdown in weak sections of the photoelectric-conversion semiconductor layer 3 other than the locations where the faults occur.
To avoid such accumulated charges and the high voltage generated by such charges, the voltage of the square-wave pulse applied to the electrodes to induce oxidation (FIG. 2) is typically restrained to levels much lower than the reverse limit voltage. Typically, a 4 V voltage pulse is applied in the reverse direction. However, such a low-voltage pulse often does not generate sufficient Joule energy, and consequently, not all of the short-circuited sections can be removed or oxidized by the generated Joule heat. At the same time, some breakdown of the non-short-circuited normal sections by discharge of the accumulated charges still occurs at certain sections where the limit voltage of the semiconductor is low.